In general, the surface light source device used for the transmission-type display device includes a light source, and a plurality of optical sheets (or optical films) that can serve to change the travel direction of the light emitted from the light source. Among these optical sheets, a light diffusing sheet that can serve to diffuse the light emitted from the light source, thus blurring an image of the light source (or rendering the light source image inconspicuous or substantially invisible), and a light collecting sheet (light condensing sheet) that can serve to collect (condense) the light travel direction in a front direction, thereby enhancing the front brightness (i.e., the brightness observed in the front direction) are included. With this surface light source device including a proper combination of the light diffusing sheet having excellent light diffusing ability and the light collecting sheet having adequate light collecting ability (light condensing ability), the transmission-type display device that can exhibit desired front brightness and an adequate angle of visibility as well as control the light source image to be more inconspicuous can be produced.
As the light diffusing sheet, the optical sheets containing proper light diffusing particles, each adapted for isotropically diffusing the light, optical sheets having a concavo-convex surface (or matted surface) and the like are well known.
As the light collecting sheets, the optical sheets, including unit shaped elements (or unit optical elements) extending along a straight line and arranged orthogonally to the longitudinal direction of each element (this arrangement manner is the so-called “linear arrangement”), are widely used. Typically, each of the unit shaped elements has a triangular, elliptical or circular shape on a cross section orthogonal to the longitudinal direction thereof. Such a light collecting sheet can serve to positively enhance the front brightness as well as can reduce in-plane variation of the front brightness attributable to specific construction of the light source and thus control the image of the light source to be more inconspicuous. Besides, this light collecting sheet can also serve to smoothly change angular distribution of the brightness centered on the front direction. That is to say, this light collecting sheet also has a light diffusing function. For instance, as disclosed in JP3309173B, in the case in which each of the unit shaped elements has the triangular cross section shape, the function for enhancing the front brightness (or light condensing function) tends to be strongly exhibited, while the function for smoothly changing the angular distribution of the brightness centered on the front direction is not exhibited so strongly. Meanwhile, in the case in which each of the unit shaped elements has the elliptical or circular cross section, the function for smoothly changing the angular distribution as well as uniforming the in-plane distribution of the brightness (or light diffusion function) tends to be strongly exhibited.
Further, JP2008-70456A discloses to form a curved surface portion on the unit shaped elements (or lens bodies) included in the light collecting sheet (light condensing sheet) such that the curved surface portion is projected from the unit shaped elements. According to the disclosure of this JP2008-70456A, the projected curved surface on the unit shaped elements can provide some light diffusing function to the light collecting sheet. Further, as reported in this JP2008-70456A, the provision of such a relatively excellent light diffusion function to the light collecting sheet is intended for reducing the number of the optical sheets to be incorporated in the surface light source device.
However, as disclosed in paragraphs [0036], [0059], [0060] and the like in the above JP2008-70456A, only the fact that the optical function (mainly the light condensing function) of the unit shaped elements (or lens bodies) and the optical function (mainly the light diffusing function) of the projected curved surfaces can be exhibited, independently of each other, is described. That is to say, from such a technique disclosed in JP2008-70456A, it cannot be expected to obtain some combined effect of the optical function of the unit shaped elements (or lens bodies) and the optical function of the projected curved surfaces. That is to say, the projected curved surfaces, as described in JP2008-70456A, are arranged on the unit shaped elements, along a direction in which this unit shaped element extends. Accordingly, depending on the weighting balance between the light condensing function and the light diffusing function, the ratio of the area that is occupied by such a projected curved surface relative to the whole surface of the unit shaped elements will be controlled. That is to say, even though the use of such a light collecting sheet disclosed in JP2008-70456A can reduce, to some extent, the number of the optical sheets to be incorporated in the surface light source device, it can be guessed from the disclosure of JP2008-70456A that this surface light source device including a light collecting sheet disclosed in JP2008-70456A cannot exhibit more excellent optical properties than that exhibited by a usual surface light source device including a usual light collecting sheet with no projected curved surface portions and a usual light diffusing sheet.
By the way, it is important to note that this light collecting sheet can change the light travel direction in a plane extending along the direction (or arrangement direction) in which the unit shaped elements are arranged. In addition, by incorporating two such light collecting sheets in the surface light source device, with the arrangement direction of the unit shaped elements in one light collecting sheet set to be orthogonal to the arrangement direction of the unit shaped elements in the other light collecting sheet, the distribution of the brightness can be well controlled, in such orthogonal two directions (typically, in vertical and horizontal directions of the sheet), on a display surface.
Further, in recent years, the so-called fly-eye lens including the unit shaped elements (or unit optical elements) arranged thereon, in different two directions, at random or regularly, has attracted significant attention in this field (e.g., see JP2006-301582A). Theoretically, the optical sheet (or fly-eye lens sheet) including this fly-eye lens can eliminate the need of using such a large number of optical sheets as described above. That is to say, by using only one optical sheet of this type, the light transmitted therethrough can be adequately collected (condensed) as well as diffused in the two directions (typically, in the vertical and horizontal directions of the sheet), on the display surface. Such reduction in the number of the optical sheets incorporated in the surface light source device is quite advantageous, because this reduction can directly lead to substantial reduction of the production cost of the surface light source device.
At present, however, either of the light collecting function (light condensing function) and light diffusing function of the fly-eye lens sheet does not yet attain a desired level. Therefore, in order to obtain these optical functions on the desired level, it is necessary to incorporate two or more fly-eye lenses into the surface light source device, thus making it rather difficult to achieve the substantial reduction of the production cost of the surface light source device.
Usually, the optical sheet is formed, by molding, with a proper radiation-curing resin (generally, a UV-curing resin). In the case of molding the fly-eye lens composed of a plurality of semi-spherical unit shaped elements (or unit lenses), a mold used for molding this fly-eye lens includes recesses having a shape closed along the whole circumference thereof upon the molding. Therefore, air may tend to enter and remain in a gap between the mold and the radiation-curing resin injected therein. In this case, an unwanted air bubble (or bubbles) is formed in the molded unit shaped elements, or otherwise a recessed defect (or defects) is formed in the surface of the molded unit shaped elements. Such a phenomenon may disable the fly-eye lens from adequately exhibiting desired and/or predetermined optical functions.